PSI - Issue 75

D. Tousse Tchamassi et al. / Procedia Structural Integrity 75 (2025) 450–456 Tousse Tchamassi / Structural Integrity Procedia (2025)

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experience severe loading conditions without any failure in both conventional and non-conventional fields, onshore and offshore applications. The development of new high strength martensitic steel grades for such connections includes the determination of their low cycle fatigue (LCF) behavior that may simulate extreme loading conditions. Manufacturing process these connections involves cold forming, so that the effect of prior plastic deformation on the LCF behavior must be assessed. Martensitic steels may exhibit cyclic softening (e.g. Guguloth et al. (2014)). On the other hand, the effect of prior plastic deformation on the fatigue behavior of steels is still controversial. A reduction in high-temperature LCF lifetime was noticed on a 9Cr-1Mo martensitic teel (Guguloth et al. (2014)). Little or no effect on fatigue crack propagation was reported in carbon steels (Nagiwara et al. (2001), Munier et al. (2010), Moço et al. (2018)). An increase in HCF lifetime was reported in a dual-phase automotive steel (Munier et al. (2010)). The effect of prior plastic strain on the LCF behavior of high strength martensitic steels is still unclear. To address this issue, the present work investigated the effect of prior tensile plastic deformation on the residual LCF lifetime and on fatigue crack initiation and propagation mechanisms of a high strength martensitic steel intended for connection manufacturing. Polished low cycle fatigue specimens were cut from an actual component and tested with or without prior tensile strain of a few percents. Specimens were also cut from regions that experienced cold forming (also up to a few percents) followed by stress relieving. Manson-Coffin curves were determined for each strain path; damage and fracture mechanisms were investigated using both 2D (scanning electron microscopy) and 3D (X-Ray tomography) techniques. Using this approach, a comparison between LCF behaviors after either simple tensile prior deformation or a more complex cold forming + stress relieving process was made. 2. Experimental procedures 2.1. Material The material of interest was a high strength martensitic steel tube with an outer diameter of 412.75 mm and an internal diameter of 375.12 mm that had been austenitized at 900°C, cooled at approximately 5°C/s and tempered at 600°C. Its chemical composition was 0.26 C - 1 Cr - 0.5 Mn - 0.5 Mo – bal. Fe (in wt%) plus minor additions (V, Nb, Al, Si). The resulting tempered martensite (Fig. 1a) showed an average prior austenite grain size of 10 µm. Its tensile properties were as follows: 0.2% proof stress 990 MPa, tensile strength 1090 MPa, maximum uniform elongation close to 5%, matching the X140 grade of API5L standard. This material is used to manufacture so-called premium integral connections (Fig. 1b). To this aim, the ends of the tubes to be connected are either expanded or restrained to fit each other. In addition to the undeformed material, the present study addressed the expanded region that was mainly deformed in tension. Preliminary finite element simulation of the expansion process gave estimates of the amount of cumulative plastic strain in the regions of interest in the final product, namely, between 2% and 4% (Fig. 1c). In the industrial application, a stress relieving heat treatment is applied to the expanded connection to give it back its original tensile properties; in the present study, however, no stress relief was applied after prior tensile deformation because plastic deformation occurring during LCF tests was expected to redistribute any internal stresses. a b c

Fig. 1. (a) Steel microstructure; (b) schematic view of the connection; (c) cumulative plastic strain (PEEQ) in the expanded part.

2.2. Mechanical testing Mechanical characterization was carried out at room temperature, in laboratory air using an INSTRON 8500 servohydraulic machine with a 250 kN load capacity. LCF specimens were taken both from the undeformed material